Due to increases in data transmission rates in telecommunications systems, crosstalk has become a significant problem. Crosstalk may be defined as stray energy which is coupled from one signal line onto an adjacent signal line by either capacitive or inductive coupling. This crosstalk results in signal noise which interferes with the purity of the signal being transmitted.
A commonly used telecommunications wiring system is unshielded twisted pair wiring wherein pairs of wires are twisted about each other. The wires in a twisted pair carry related signals and are thus known as signal pairs. Crosstalk is most likely to occur between adjacent wires in different signal pairs.
Each of the wires in a signal pair carries an equal but opposite signal; that is, the wires carry signals of the same magnitude which are respectively positive and negative. Since these signals are equal but opposite, they generate fields that are equal but opposite. In a twisted pair these equal and opposite fields cancel each other. Thus, little or no crosstalk can occur between one twisted pair and an adjacent twisted pair.
Crosstalk in unshielded twisted pair wiring systems primarily arises in the electrical connectors which provide an interface between successive runs of cable in a system. Industry standard electrical connectors for communications systems include modular plugs and jacks, and AT&T style insulation displacement connectors. These connectors have pins or terminals which are spaced closely together and parallel to each other. This close and parallel arrangement is conducive to crosstalk between adjacent lines in different ones of the signal pairs. Since crosstalk increases logarithmically as the frequency of the signal increases, high speed data communications systems have been increasingly plagued by crosstalk which occurs within the electrical connectors.
The connectors such as modular jacks are commonly mounted on a circuit board which interconnects through a card edge connector to the wiring system. The circuit board carries circuit lines or traces which extend between one set of terminals on the board which are matable with the modular jack, and another set of terminals on the board which are matable with the card edge connector.
Prior art techniques for reducing crosstalk include arrangements of the circuit traces on the board in such a manner that energy coupling in adjacent traces will cancel the effects of energy coupling which occurs in the modular plug and jack. U.S. Pat. No. 5,299,956 discloses an arrangement wherein the traces on the board are routed in a pattern that is opposite in polarity to the pattern that produces crosstalk in the plug and jack. However, it has been found that this technique results in higher structural return loss because of the impedance mismatch between the connector system and the cable.
U.S. Pat. No. 5,310,363 discloses a crosstalk reduction method which avoids the higher return loss. The technique involves adding an additional trace to the board. The additional trace interconnects two of the same terminals which are interconnected by one of the other traces. Thus, two of the traces each carry an identical signal. These traces are routed so that the signal on one trace provides proper pair balance and impedance, and the signal on the other trace provides increased coupling with a trace in one of the other pairs for crosstalk reduction.
It in now proposed to reduce crosstalk by providing an additional trace which is not ohmically connected to any of the other traces, or to ground. The non-ohmically connected trace is routed closely adjacent to portions of two or more traces in different signal pairs, thereby coupling energy from each trace to the other so that crosstalk is reduced.